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Newswise — Aerobic exercise triggers a reward system in the body of mammals built for endurance – like humans – but not other creatures, a new study from the University of Arizona and Eckerd College says.

The findings suggest that natural selection used the pleasures of “runner’s high” to motivate endurance exercise in humans and other animals that run over long distances.

"Aerobic activity has played a role in the evolution of lots of different systems in the human body, which may explain why aerobic exercise seems to be so good for us," says David Raichlen, assistant professor of anthropology at the University of Arizona.

Most human athletes experience the infamous 'runner's high' after exertion, which is caused by endocannabinoid signaling in the so-called 'reward centers' of the brain. Chemically related to the active ingredient in cannabis, endocannabinoids are substances produced by the body that act as neurotransmitters.

Riachlen teamed up with Greg Gerdeman, assistant professor of biology at Eckerd College in St. Petersburg, Florida, and Andrea Giuffrida the University of Texas in San Antonio to examine how exercise influences the endocannabinoid levels of two mammalian natural athletes – humans and dogs – and a sedentary species like ferrets.

Blood samples were collected by recreational runners and pet dogs, trained to walk or run on a treadmill, before and after exercise. The less-cooperative ferrets also had blood samples collected after exercise and during rest.

Analysis of the samples for endocannabinoid levels found that the concentration of one endocannabinoid – anandamide – spiked in the blood of dogs and humans after a brisk run. Ferrets did not enjoy the same effect. In humans, the measured levels of anandamide correlated closely to an increase in positive emotional feelings described by the subjects after the run – a good mood indicative of a healthy “runner’s high.”

Says Raichlen, "These results suggest that natural selection may have been motivating higher rather than low-intensity activities in groups of mammals that evolved to engage in these types of aerobic activities."

Gerdeman adds that if their hypothesis is true, “the result of this anandamide-inspired motivation to run was the evolution of an ‘endurance athlete phenotype’ that played a major role in the survival and reproductive success of our Homo sapiens ancestors.”

“This is only one of several lines of evidence suggesting that endocannabinoid signaling may underlie some of the physiological benefits of exercise,” says Gerdeman. “Future studies could work to elaborate what kinds of specific exercise regimens might maximize therapeutic outcomes of activating the cannabinoid receptors, such as treatment of depression, for example.”

The study, "Wired to run: exercise-induced endocannabinoid signaling in humans and cursorial mammals with implications for the 'runner's high'" was published in The Journal of Experimental Biology.